A large rotary electric machine typified by a turbine generator needs to have high insulation performance that passes a withstand voltage test performed before shipment and does not cause defects such as an insulation abnormality for an operation period of several tens of years. Particularly in a stator coil to which a high voltage is applied, the structure described below is generally adopted to suppress partial discharge and insulation abnormalities caused by partial discharge during a withstand voltage test or normal operation.
In the basic structure of a stator coil, a main insulation layer is formed around a coil conductor. This coil conductor includes, for example, a bundle of copper elemental wires. In addition, the main insulation layer is formed by winding mica tapes having very good corona-resistant discharge characteristics around the coil conductor, impregnating the mica tapes with heat-hardening resin such as epoxy resin, and curing the heat-hardening resin.
A part of the stator coil is accommodated in a slot of a stator iron core and an end part thereof extends outside the slot. The part of the stator coil accommodated in the slot of the stator iron core and the part partially extending outside the slot from this part are provided with a low resistance corona shield layer having semi-conductivity on the outer peripheral part of the main insulation layer. This low resistance corona shield layer makes close contact with the stator iron core having a ground electric potential and has the function of suppressing discharge within the slot by setting the electric potential on the outermost layer of the stator coil to the ground electric potential.
On the other hand, the part of the stator coil extending outside the slot is generally referred to as a coil end. The surface potential of the coil end sharply rises from the end part of the low resistance corona shield layer covering the part partially extending outside the slot toward the outside in the longitudinal direction of the coil. This sharp difference of the surface potential may cause creepage discharge at the coil end.
A nonlinear resistance layer partially overlapping with the end part of the low resistance corona shield layer is provided to suppress such a sharp rise in the surface potential at the coil end. In the nonlinear resistance material constituting this nonlinear resistance layer, the electric resistivity reduces nonlinearly as the electric field strength applied to the material increases. In the nonlinear resistance layer provided on the surface of the coil end, when the surface potential difference (electric field strength) of the stator coil increases to a certain level or more, the electric resistivity of the nonlinear resistance layer reduces. As a result, current flows to the low resistance corona shield layer having the ground electric potential in the nonlinear resistance layer and suppresses a sharp rise in the surface potential of coil end, thereby enabling suppression of creepage discharge. This function of suppressing occurrence of creepage discharge of the nonlinear resistance layer is referred to as the electric field relaxation function.
The nonlinear resistance material used for a large rotary machine generally includes silicon carbide (SiC) particles mixed with insulating resin. The nonlinear resistance layer is formed by, for example, shaping such a material in a semi-hard state like tapes, winding the material around the surface of the main insulation layer, which is a basic structure of the stator coil, and thermally curing the material or by applying such a material in a paint-like state onto the surface of the main insulation layer and drying the material. The electric resistivity varies greatly in the nonlinear resistance layer formed in this way, thereby increasing production variations in the electric field relaxation function. In order to improve the electric field relaxation function, the nonlinear resistance layer having a large electric resistivity and the nonlinear resistance layer having a small electric resistivity have been used together.
For example, a plurality of nonlinear resistance layers is laminated with each other sequentially toward the outside of the coil end so that the outside nonlinear resistance layer (upper layer) has an electric resistivity and formation length larger than the nonlinear resistance layer (lower layer) close to the slot (see PTL 1, for example). In another method, a plurality of nonlinear resistance layers having different electric resistivities are laminated with each other so that the nonlinear resistance layer with a smaller electric resistivity has a smaller formation length (see PTL 2, for example).